I just did a taste-test comparison between the two last night (I have some SURGE hidden away I bought in 2004), so I'm personally confident with my findings.
I think the extra heaviness of VAULT comes from the "stuff" they had to add in (artificial flavoring, more sugar) to offset the bitterness of the extra caffeine.
But for me the difference is really subtle. When I tried the SURGE last night, my first impression was, "This tastes like VAULT." After guzzling some of it and VAULT< I moved on to dnL, Sun Drop, and MDX, and none of those taste anything remotely like SURGE and VAULT. It's pretty clear to me that Coke, inadvertantly or not, stayed away from cloning its competitors and ended up with yet another tweak of SURGE.
-HM
"I see a Hollow Man, gun in hand, it points my way." - The Cult